Analytical 



at once "a few" ml. of standard silver nitrate solution. The 

 mixture is shaken, and allowed to stand for five minutes. 

 Five drops of a lo per cent, solution of calcium chloride and 

 ten drops of a 20 per cent, solution of ammonium carbonate 

 are then added, in order to form a precipitate of calcium 

 carbonate to carry down suspended silver. The mixture 

 is filtered, and the filtrate tested with silver nitrate. If 

 appreciable reduction occurs, a new experiment is begun, 

 using 2 to 5 times as much standard silver nitrate solution as 

 first. The reduction test is repeated, and the whole thing is 

 gone through several times, if necessary, until no reduction 

 occurs in the filtrate. This gives an upper limit to the volume 

 of standard silver nitrate. The experiments must again be 

 repeated ab initio with successively slightly smaller doses of 

 standard silver nitrate until no reduction occurs on the 

 addition of silver nitrate to one portion of the filtrate, and also 

 no or very slight cloudiness is revealed on the addition of 

 excess hydrochloric acid to another portion. Each ml. of 

 standard silver nitrate solution that has produced this limiting 

 result is equivalent to 4*124 mg. of homogentisic acid. The 

 method is probably capable of simplification. 



Indole-acetic Acid. The Urorosein Test 

 A number of chemical reactions are available for revealing 

 the presence of indole-acetic acid. They have been described 

 in the literature as tests for the substance in urine, or in fairly 

 concentrated pure solution. Some tests for the pure substance 

 follow. 



E. Salkowski's tests date from 1884 and are the best known. 

 (i) If a o-i per cent, neutral solution of the acid is treated 

 with a little very dilute ferric chloride solution, the mixture 

 becomes cloudy; in transmitted light it appears violet, but 

 greyish in reflected light. On careful acidification a violet-grey 

 precipitate is formed, which gives a blue solution in ethyl 

 alcohol. 



If after adding ferric chloride and acid, more ferric chloride 

 is added and the mixture is boiled, a cherry-red colour is 

 produced. The colour can be removed by extraction with 



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